In the related art, a light beam scanner for scanningly deflecting a light beam, such as a laser beam, is used in various optical apparatuses, such as barcode readers, laser printers and head-mounted displays, or as a light pickup element of an optical input device for infrared cameras or the like. In connection with this type of light beam scanner, there has been proposed a light beam scanner designed to rotationally oscillate a micromirror prepared using silicon micromachining techniques.
For example, FIG. 14 shows a light beam scanner having a silicon micromirror which is disclosed in Japanese Patent Laid-Open Publication No. 11-52278 (hereafter “JP '278”). This light beam scanner is fabricated using silicon micromachining techniques, so as to have the total area on the order of several millimeters square. A support substrate 1 is formed of a rectangular thick plate. The support substrate 1 has a central region formed as a concave portion 1a, and a mirror 2 formed of a silicon thin film and supported by the support substrate 1 so as to be located inside the concave portion 1a. This mirror 2 is integrally formed with two torsion bars 3a, 3b protruding, respectively, from two opposite edges thereof The distal ends of the torsion bars 3a, 3b are each fixed to the support substrate 1, and two pads 4a, 4b are electrically connected, respectively, to the distal ends of the torsion bars 3a, 3b. This structure allows the mirror 2 to be rotationally oscillated in a direction perpendicular to an in-plane direction of the mirror 2 in conjunction with a torsional deformation of the torsion bars 3a, 3b. At least a peripheral region or a surface of the mirror 2 is subjected to impurity ion-implantation or diffusion, or coated with an aluminum or silver film or an organic thin film having electrical conductivity. This peripheral region serves as an electrode portion 5 having electrical conductivity.
Further, two fixed electrodes 7a, 7b are disposed on a top surface of the support substrate 1 through an insulator 6, respectively, on opposite sides of the concave portion 1a. These fixed electrodes 7a, 7b are made of a conductive material consisting of a semiconductor or organic material. Each inner edge of the fixed electrodes 7a, 7b is disposed close to the electrode portion 5 at a corresponding one of opposite lateral edges of the mirror 2, so that a capacitor is formed between the electrode portion 5 and each of the fixed electrodes 7a, 7b. 
When a prescribed voltage is applied between a pad 8a of the fixed electrode 7a and each of the pads 4a, 4b of the torsion bars 3a, 3b, the prescribed voltage is also applied to the electrode portion 5 of the mirror 2 connected to the pads 4a, 4b, to allow electric charges to be stored in the surface of the fixed electrode 7a and the surface of the electrode portion 5 of the mirror 2 in opposite polarities, so as to form a capacitor therebetween. Thus, an electrostatic attraction acts between the fixed electrode 7a and the electrode portion 5 of the mirror 2 to induce rotation of the mirror 2. Subsequently, when the mirror 2 is returned to its original position, a prescribed voltage is applied between the fixed electrode 7b and the electrode portion 5 of the mirror 2 to rotate the mirror 2 in the opposite direction. The above operation is repeated to allow the mirror 2 to perform a rotationally oscillating movement in which the mirror 2 is rotated up to respective maximum counterclockwise and clockwise rotational positions repeatedly and alternately.
There is also a related art light beam scanner for rotationally oscillating a micromirror fabricated using silicon micromachining techniques as disclosed in Japanese Patent Laid-Open Publication No. 10-197819 (hereafter “JP '819”). As shown in FIG. 15, this light beam scanner comprises a plate-shaped micromirror 1 for reflecting light, a pair of rotating support members 2 aligned in a straight line to support opposite edges of the micromirror 1, a frame 3 surrounding the periphery of the micromirror 1, and a piezoelectric element 4 for producing a translational movement of the frame 3. The center of gravity of the micromirror 1 is located at a position deviated from the straight line connecting the pair of rotating support members 2.
The piezoelectric element 4 elongates and shrinks, or vibrates, in a Z-axis direction, in response to a prescribed voltage applied thereto. This vibration is transmitted to the frame 3. Thus, the micromirror 1 has a relative movement with respect to the driven frame 3. Specifically, when a Z-axis directional vibration component is transmitted to the micromirror 1, a rotational moment is produced in the micromirror 1 around the rotating support members 2 on an X-axis, because the micromirror 1 has mass components laterally asymmetric relative to an axial line defined by the rotating support members 2 on the X-axis. In this way, the translational movement of the frame 3 produced by the piezoelectric element 4 is converted to a rotational movement of the micromirror 1 around the rotating support members 2 on the X-axis.
The light beam scanner in JP '278 is fabricated using silicon micromachining techniques to have a total area on the order of several millimeters square. Moreover, it is required to have the electrode portion 5 formed in at least a peripheral region or a surface of the mirror 2, the pads 4a, 4b connected to the torsion bars 3a, 3b, the fixed electrodes 7a, 7b disposed, respective, on opposite lateral sides of the top surface of the support substrate 1 through the insulator 6, and the pads 8a, 8b connected to the fixed electrodes 7a, 7b. 
The requirement of having the electrode portion 5 formed in at least a peripheral region or a surface of the mirror 2, the pads 4a, 4b connected to the torsion bars 3a, 3b, the fixed electrodes 7a, 7b disposed, respective, on opposite lateral sides of the top surface of the support substrate 1 through the insulator 6, and the pads 8a, 8b connected to the fixed electrodes 7a, 7b, causes structural complexity, and increases in factor causing failures, lost manufacturing time and increased cost.
The light beam scanner in the JP '819 is designed to convert the translational movement of the frame 3 produced by the piezoelectric element 4 to the rotational movement of the micromirror 1 around the rotating support members 2 on the X-axis. Thus, the center-of-gravity position of the micromirror 1 must be deviated relative to the axial line defined with respect to the rotating support members 2. Moreover, a thickness of this light beam scanner is inevitably increased in not only the X-Y directions but also the Z-axis direction, which causes difficulties in achieving a thin structure.